The present invention relates to a process for the preparation of unsaturated alcohols. More particularly, this invention relates to the preparation of .alpha.,.beta.-ethylenically unsaturated primary alcohols by catalytic hydrogenation of the corresponding .alpha.,.beta.-ethylenic aldehydes.
Among all the processes for obtaining .alpha.,.beta.-ethylenic alcohols described in the literature, there are to be found several processes for the catalytic hydrogenation of the corresponding .alpha.,.beta.-ethylenic aldehydes. The critical point of the hydrogenation resides in the selectivity of the catalyst, because the ethylenic double bond which is in the conjugated position relative to the carbonyl group can also become hydrogenated under the conditions of reduction of the carbonyl group. The selective hydrogenation of an .alpha.,.beta.-ethylenic aldehyde to the corresponding .alpha.,.beta.-ethylenic alcohol is rendered more difficult still when one or more ethylenic double bonds are present in a conjugated or non-conjugated position, along side the unsaturation in the .alpha.,.beta.-position relative to the formyl or carbonyl group to be reduced.
Various metal catalysts have been proposed for carrying out this hydrogenation; platinum, in its various forms, has for a long time been considered a particularly suitable catalyst. This catalyst can be, in particular, platinum black or mixtures of platinum black and PtO.sub.2 obtained by pre-reduction of platinum dioxide at ambient temperature under a low pressure of hydrogen compare W. F. Tuley and R. Adams, J. Amer. Chem. Soc., 47, pp. 3061 to 3068 (1925)! or metallic platinum deposited on a support, such as charcoal or calcium carbonate (compare U.S. Pat. No. 3,284,517).
However, it is noted that use of these platinum-based catalysts requires the conjoint use of promoters if the formyl or carbonyl group of the .alpha.,.beta.-ethylenic aldehyde is to be reduced selectively. Thus, Tuley and Adams employ ferrous chloride or ferrous sulphate (about 0.2 gram atom of Fe per gram atom of platinum), which may or may not be mixed with zinc acetate (about 0.03 gram atom of Zn per gram atom of platinum), while in the above-mentioned U.S. patent, the use of ferrous chloride mixed with silver nitrate (about 0.4 gram atom of Fe and 0.06 gram atom of Ag per gram atom of platinum) is proposed. The reaction wherein the aldehyde substrate is hydrogenated is carried out under hydrogen pressure, at a temperature ranging from 25.degree. C. to 100.degree. C., in general working in a hydroxylic solvent such as a lower saturated aliphatic alcohol, which contains about 0.5 to 2 g. of metallic platinum per mol of aldehyde to be reduced.
The performance achieved by means of the above-mentioned processes is satisfactory; for example, starting from cinnamaldehyde, cinnamyl alcohol is formed predominantly, the remainder (of the product) consisting of 3-phenyl-propan-1-al and/or 3-phenyl-propan-1-ol, and the selectivity of the hydrogenation, measured by the yield of cinnamyl alcohol relative to aldehyde consumed, is greater than 85%; it can reach 90% and even exceed this value.
However, employing noble metals such as platinum is only desirable from an economic point of view if it is possible to recycle the catalyst to the reaction zone without excessively altering the performance obtained with the fresh catalyst. The processes proposed do not offer satisfactory solutions in this respect. In fact, in studying the possibility of recycling the above-mentioned catalyst systems it has been found that, starting from cinnamaldehyde, there is a sharp drop in the catalytic activity at the first attempt at recycling, which manifests itself particularly in a significant lengthening of the reaction time required to achieve a degree of conversion of the aldehyde equivalent to that obtained when the catalyst is first employed, or in a lowering of the degree of conversion if one chooses the same duration for the hydrogenation. This loss in efficiency of the catalyst being recycled thus brings about a reduction in the productivity of the production apparatus and, consequently, reduces the industrial value of such hydrogenation processes.
It was thus desirable to provide a process of hydrogenation in the presence of a platinum-based catalyst which makes it possible to overcome the disadvantages of the prior processes with regard to recycling of the catalyst, while preserving the above-mentioned advantages, especially excellent selectivities in respect of the product of hydrogenation of the formyl group.
There has now been found, and it is this which forms the subject of the present invention, a process which satisfactorily meets this objective.
It is, therefore, an object of the present invention to provide a process for reducing .alpha.,.beta.-ethylenic aldehydes to the corresponding .alpha.,.beta.-ethylenic primary alcohols, which process overcomes the disadvantages of the prior art.
It is also an object of the present invention to provide a catalytic reduction process whereby the catalyst can be re-used or recycled in the process without a substantial diminution in performance of selectivity in reducing the carbonyl or formyl group without concomitant reduction of the ethylenic unsaturation.
It is a further object to provide a catalytic reduction process which does not require a promoter for the platinum catalyst.
Other objects will be apparent to those skilled in the art from the present description.